The present invention relates generally to a superconducting circuit incorporating Josephson devices, and more particularly to a DC flux parametron circuit and a memory cell as well as a memory circuit in which makes use of the superconducting circuit.
A switching circuit based on the Josephson effect has been well known in the art, as typically exemplified by a quantum interference circuit and a direct coupled circuit or the like. The quantum interference circuit is a switching circuit constituted by employing superconducting loops including two or more Josephson junctions. For particulars, reference may be made to "IBM Research and Development" Vol. 24, No. 2 (1980) and Japanese unexamined patent application laid-open No. 12965/1979. The direct coupled circuit is a non-quantum interference device in which current is injected at the Josephson junction. As a typical type of direct coupled circuit, there can be mentioned a direct coupled logic circuit (DCL circuit in abbreviation), details of which are disclosed in "International Electron Device Meeting, Technical Digest" (1979). The prior art switching circuits employing the Josephson devices are disadvantageous in that the gain is as low as about 2 (two). Further, because of the necessity to maintain constant a bias current flow in the circuit, power consumption per unity circuit amounts to ca. 1 to 10 .mu.W. The Josephson device is inherently destined for operation under cryogenic conditions and is ordinarily used in the stage immersed in liquid helium. In consideration of heat of vaporization of liquid helium, the power consumption of 1 to 10 .mu.W per unity circuit means a great amount of heat generation, providing difficulty and obstacle in implementing logic circuits and systems of a high integration density by using the Josephson devices according to the prior art techniques, which is a disadvantage.
In the case of the quantum interference circuit mentioned above, a single superconducting loop is composed of two Josephson junctions and an inductor. Consequently, resonance circuits are formed by junction capacitances of the two Josephson junctions and the inductor, giving rise to a problem that may result in erroneous operation because of the resonance effect. In conjunction with this, the aforementioned literature "IBM Research and Development", Vol. 24, No. 2 (1980) and Japanese unexamined patent application laid-open No. 12696/1979 teach connecting a resistor to the inductance element forming a part of the resonance circuit for the purpose of suppressing the resonance.
Further, as a switching device based on the Josephson effect which exhibits a high gain and a reduced power consumption, there has been porposed a DC flux parametron circuit (also referred to as DCFP circuit in abbreviation). The principle of the DCFP circuit is disclosed in detail in a literature titled "Josephson Electronics", pp. 1-3, a collection of lectures in symposium held by the Institute of Physics and Chemical Research on Mar. 16, 1984 in Japan and Japanese unexamined patent application laid-open No. 14342/1984. These references are incorporated herein by reference.
The DCFP circuit is excellent in performance over the prior art quantum interference circuit in that the power consumption of the former is extremely low, typically on the order of 1/1000 of that of the latter, and that the former can assure a switching speed of less than 10 ps. The DCFP circuit can enjoy remarkably wide range of applications such as those corresponding to logic circuits and memory circuits. An application of the DCFP to a memory cell is disclosed in Goto et al's article contained in the aforementioned literature "Josephson Electronics" pp. 96-102. This reference is also incorporated herein by reference. It should however be mentioned that the switching circuit constituted by the DCFP circuit is also composed of two Josephson junctions and an inductor. As a consequence, the junction capacitances of the two Josephson junctions cooperate with the inductor to form two resonance circuits whose resonances can effect in erroneous operation, which is a disadvantage. Similarly, in the case of the memory cell constituted by using the DCFP circuit, there are formed three resonance circuits for the three Josephson elements, respectively, wherein resonances of these circuits may effect in erroneous operation of the memory cell.